Brain tumors are an important cause of morbidity and mortality in children and adults. Conventional treatments, including radiation therapy and chemotherapy, are often ineffective and efforts to identify new strategies in brain tumor treatment are warranted. Cell cycle regulation check points represent a new class Of potential targets for anti-tumor treatment. Progression of proliferating cells through the cell cycle is regulated by the cyclin family of proteins and their corresponding cyclin-dependent kinases. Recent evidence indicates that cyclin A and B1, and cdc2 kinase, which regulate cellular progression through G2 to mitosis (M), also mediate cellular responses to DNA damage, programmed cell death (i.e., apoptosis), and differentiation. The function of cyclins and their associated kinases in brain tumor cell cycle regulation is poorly understood; however, our preliminary studies clearly implicate G2/M check points in human glioma and medulloblastoma cell death. To evaluate G2/M check point regulation and assess their potential as targets in brain tumor therapy we propose: 1. To identify the essential G2/M check point regulators in human glioma and medulloblastoma cells; 2. To increase chemotherapy- and radiation-induced cytotoxicity in human brain tumor cell lines by altering G2/M check point regulators; 3. To identify G2/M check point regulators which mediate apoptosis and/or differentiation; and 4. To induce apoptosis or differentiation by altering G2/M check point regulators identified above. Identification of the G2/M check point regulators mediating the responses of cells to DNA damage, programmed cell death, and terminal differentiation will be achieved by selectively decreasing cyclin expression in tumor cells transfected with cyclin antisense expression vectors, followed by analysis of the changes in progression through the cell cycle, sensitivity to the cytotoxic effects of radiation or chemotherapy, and apoptosis or differentiation. Once cyclins important in regulating cell death and differentiation have been identified, we will induce their overexpression by transfecting tumor cells with cyclin A or B1 sense expression vectors. This strategy may inhibit cell growth by exploiting cellular mechanisms for apoptosis or terminal differentiation or by over-riding cellular mechanisms that permit repair of DNA-damage caused by radiation or chemotherapy. We anticipate that these studies will provide a foundation for future pre-clinical and clinical studies of anti-tumor therapies which target G2/M cell cycle check point regulation.